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Latin american journal of aquatic research

versión On-line ISSN 0718-560X

Lat. Am. J. Aquat. Res. v.38 n.3 Valparaíso  2010

http://dx.doi.org/10.4067/S0718-560X2010000300007 

Lat. Am. J. Aquat. Res., 38(3): 427-437, 2010
DOI: 10.3856/vol38-issue3-fulltext-7

RESEARCH ARTICLE

 

Preference, tolerance and resistance responses of Poecilia sphenops Valenciennes, 1846 (Pisces: Poeciliidae) to thermal fluctuations

Respuesta de preferencia, tolerancia y resistencia de Poecilia sphenops Valenciennes, 1846 (Pisces: Poeciliidae) a fluctuaciones térmicas

 

Mónica Hernández-Rodríguez1 & L. Fernando Bückle-Ramirez1

1 Centro de Investigación Científica y Educación Superior de Ensenada (CICESE) Carretera Ensenada-Tijuana 3918, Baja California, Mexico

Dirección para Correspondencia


ABSTRACT. Poecilia sphenops was acclimated to two thermal fluctuations, each following a symmetrical and asymmetrical cycle. The critical maximum temperatures of the fish were significantly different (P < 0.001) between the thermal fluctuations: 40°C for the 20-29°C thermal cycle and exceeding 42°C for the 26-35°C cycles. Fish acclimated to the 20-29°C thermal cycle were more resistant to cold, with a critical thermal minimum of 10°C, unlike specimens of the 26-35°C regime, with a critical minimum of 11.7°C. In both cycles, the upper incipient lethal temperature interval was 38.8 to 39.5°C and the lower incipient lethal temperature interval was 10.8 to 11.8°C. Females preferred temperatures between 30 and 31°C, in all but the 26-35°C symmetric cycles. Males preferred a temperature between 23.8 and 24.2°C (symmetrical cycle). The high and low avoidance temperatures of male and female fish acclimated to the 20-29°C cycle were, respectively, 14 and 16°C, compared to the 26-35°C treatment, in which case these values were 8.4 and 11.4°C. The results indicate that P. sphenops is perfectly adapted to the marked seasons of rain and drought of their habitat.

Keywords: thermal fluctuations, behaviour, fishes, Poecilia sphenops.


RESUMEN. Poecilia sphenops fue aclimatada a dos fluctuaciones térmicas cada una con un ciclo simétrico y asimétrico. La temperatura crítica maxima de los peces fue significativamente diferente (P < 0.001) entre las fluctuaciones y se observó a 40°C en el ciclo 20-29°C; para el ciclo 25-35°C fue mayor a 42°C. Los peces aclimatados a la fluctuación 20-29°C fueron más resistentes al frío con una temperatura crítica mínima de 10°C, en contraste a los 11.7°C del régimen 26-35°C. En ambos ciclos la temperatura letal incipiente superior tuvo un intervalo de 38.8 a 39.5°C y la temperatura letal incipiente inferior fue de 10.8 a 11.8°C. La temperatura preferida de las hembras fue de 30 a 31°C, con excepción del ciclo simétrico 26-35°C. La temperatura preferida de los machos aclimatados al ciclo simétrico fue de 23.8 a 24.2°C y de 27.4 a 29.4°C en el ciclo asimétrico. Las temperaturas de evitación de los machos y las hembras aclimatados a la fluctuación 20-29°C tienen un intervalo de 14 a 16°C comparado con el intervalo de 8.4 all .4°C del tratamiento 26-35°C. Los resultados indican que P. sphenops está perfectamente adaptado a las marcadas estaciones de lluvia y sequía de su habitat.

Palabras clave: fluctuaciones térmicas, comportamiento, peces, Poecilia sphenops.


INTRODUCTION

Aquatic organisms react in the presence of thermal variations avoiding lethal temperatures and preferring others, which allow them to survive and reproduce (Brett, 1956). In a graphic model enclosing a tolerance and resistance area, Fry (1947) and Brett (1956) demonstrated the existing relationship among the organism's responses (preferred, avoided, lethal and critical temperature) and the constant acclimation temperature.

The tolerance area comprises the low and high avoided temperatures that are less frequented by the organisms as well as the preferred temperature or final preferendum described by Fry (1947). This area is limited by the lower and upper incipient lethal temperatures (LILT and UILT) defined as the temperature beyond which 50% of the population survives an indefinitely long exposure (Fry, 1947).

Thermal tolerance of an organism is based on the lethal incipient temperature; however, fish avoid temperatures exceeding the thermal preference limited by the low and high avoidance temperatures delimiting the thermal preference area (Giattina & Garton, 1982). This zone reflects the optimum temperature for the biological processes, which occur inside the thermal interval of the species habitat where they become more efficient near the final preferendum (Crawshaw, 1977).

The resistance area is limited by the critical thermal minimum and maximum (CTMin and CTMax), characterized by the interaction of temperature and time. These responses are used as stress and adaptation indicators for invertebrate and vertébrate poikilotherms (Paladino et al., 1980).

Research has been carried out in different species to relate the effect of fluctuating temperatures with different process such as, the thermoregulatory behavior, thermal tolerance, metabolism, growth rate and mortality, development time, maturity among others (Feldmeth et al., 1974; Orto, 1974; Hokanson, 1977; Diana, 1984; Hernández-Rodríguez et al., 2002; Hernández-Rodríguez & Bückle-Ramírez, 2002; Rahel, 2003; Portner et al., 2004; Carveth et al., 2006; Widmer et al., 2006). Their results are very important because several species of the same environment were contrasted. In the present work we only used Poecilia sphenops as a target species particularly by his extended geographical distribution.

With base in the limits of the temperature polygon proposed by Fry (1947) and Brett (1956), the behavior response of an organism, is a tool that allows predicting the individual's behavior in a thermolabil environment (Anguilletta, 2009), its limits, as well as the optimum temperature interval in which the species can be cultivated, and a guidance for evaluating and recommending temperature regimens (Armour, 1991). This work evaluated and compared the experimental responses of Poecilia sphenops thermal fluctuations, versus previous investigations carried out in the same species (Hernández-Rodríguez & Bückle-Ramírez, 1998; Hernández-Rodríguez et al., 2002) acclimated to constant temperatures regimes to predict their environmental performance capacity.

MATERIALS AND METHODS

P. sphenops 2 to 6 cm total length was collect in Piedra Azul dam in the Municipality of Teotitlan del Valle (17°02'N, 96°30'W), Oaxaca, Mexico. In the laboratory, two thousand fish were distributed in three fiberglass circular tanks 380 L each where they stayed in quarantine at the site's temperature (26°C) and exposed to natural photoperiod. After quarantine, organisms were exposed to fluctuating acclimation temperatures to encompass the prevailing temperature of the dam and particularly the extreme temperature of the shallow area, each one constituted by two independent cycles. We choose the symmetric cycle (SC) proposed by Feldmeth and Stone (1974) used in Cyprinodon nevadensis amargosae and we add an asymmetric cycle (AC) to identify the change in species tolerance and resistance. The criteria to establish the thermo cycles were based in the environmental thermal fluctuations. The populations of P.ephenops that inhabit the dam in Oaxaca tolerate prolonged periods of drought and rain. In June-October (Summer-Autumn) prevail high temperatures and lower in November-December (late Autumn-Winter) averaging 19 to 28°C throughout the year. In the shallow areas of the dam where the reproducers are located (Barón, 2001) higher temperatures have been register (35°C).

Average weight and standard length of females and males acclimated to the 20-29 and 26-35°C both, exposed to the SC and AC are shown in Table 1. Four circular tanks of 380 L were used, two of them were modified to genérate cycles of 24 h (Fig. 1). The 20-29°C thermal fluctuation (TF) applied to fish (N = 400) (aprox. 2 fish per litre) of the symmetrical cycle (SC) of 5-7-5-7 h, began with the heating period 20-29°C in 5 h, then temperature was maintained at 29°C for 7 h, followed by a cooling period 29-20°C in 5 h keeping temperature at 20°C for 7 h before beginning the new cycle. Fish (N = 400) of the asymmetric cycle (AC) 4-10-5-5 h, experienced the heating period 20-29°C in 4 h, followed by 10 h at 29°C, followed by a cooling period 29-20°C in 5 h and maintaining temperature at 20°C for 5 h before beginning the new cycle.


Two tanks, each one with a 1000 watts heater connected to an automatic clock controller (ChronTrol ) began the fish heating period at the established time. Heating began at 6:00 h A.M. and when the water temperature reached 29°C, the controller kept it constant until the starting of the cooling period, which was made replacing 17°C water firom the storage tank (Fig. 1b). Water replacement for the SC was 550 mL min-1 and 660 mL min-1 for the AC. In the tanks, a central drainage functioned as level controller (Fig. 1); the surplus water was received in one of the lower tanks and returned again to the 17°C storage tank.


Figure 1. System used in the acclimation oíPoecilia sphenops to fluctuating temperatures. A: acclimation tanks, B; tank for water supply (17°C), C: reception tank for the water of the acclimation tanks, D: cooling unit (chiller), E: motor-pump, F: submerged motor-pump, G: control clock (timer, ChronTrol ®); H: electronic regulator of the thermocouple and the heater, I: thermocouple, J: 1000 watts titanium heater, K: air, L: isolating cover.

Figura 1. Sistema usado para la aclimatación de Poecilia sphenops a temperaturas fluctuantes. A: estanque de aclimatación, B: estanque para el suministro de agua (17°C), C: estanque de recepción para el agua del estanque de aclimatación, D: unidad de enfriamiento (chiller), E: motobomba, F: motobomba sumergible, G: reloj control (timer, ChronTrol ®), H: regulador electrónico de la termocupla y el calentador, I: termocupla, J: calentador de titanio de 1000 vatios, K: aire, L: tapa aislante.

We applied the same cycle's design and the same amount of fish per tank for the 26-3 5°C TF. For these cycles, water temperature in the storage tank was cooled to 10°C (Neslab RTE-220). Timing for heating and cooling periods was the same as for the 20-29°C TF.

The organisms of the two TFs were fed ad libitum twice a day with catfish food containing 35% protein. Daily, faeces, death and sick organisms were subtracted and measured the dissolved oxygen concentration (Oxymeter YSI 50B), pH (Orion SA 23), ammonium concentration (Nessler method), salinity (Lapcomp SCT-100) and water hardness (Hach 5B) as CaCO3. In each tank a thermograph registered the temperatura every 30 min for 30 days in order to know the variation of the fluctuating acclimation temperatures. The measured physico-chemical water parameters for the 20-29 and 26-3 5°C TFs are presented in Table 2.


The experiments initiated after 30 days acclimation period of adjustment to the fluctuating temperatures and 12 h light: 12 h darkness photoperiod. Organisms were not fed 24 h prior to the experiment to avoid post-digestive processes to interfere with behavior responses.

Critical thermal maximum and minimum (CTMax and CTMin), upper and lower incipient lethal temperatures (UILT and LILT), and preferred (PT) and avoided temperatures (AT) experiments were carried out with four repetitions. All experiments began one hour after fish had reached the cycle's respective maximum temperature.

The resistance of P. sphenops was evaluated with CTMax trials, by increasing the water temperature 1°C min-1 according with Paladino et al. (1980) and Lutterschmidt & Hutchison (1997) until the responses, muscular spasms (MS) and loss of equilibrium (LE) were observed. Experiments began with the temperature of the aquarium at the highest cycle temperature (29 or 35°C). The 10 fish used for each experiment of the 20-29°C TFs (N = 40) had a weight of 0.76 ± 0.12 g and a standard length of 2.99 ± 0.12 cm. Fish of the 26-35°C TFs (N = 40) had a weight of 1.08 ± 0.23 g and a standard length of 2.44 ± 0.20 cm.

Ten fish were used for each CTMin experiments, the weight was 1.31 + 0.37 g and their standard length 3.53 + 0.29 cm. Behavior responses of sporadic quick activity with muscular spasms (SA+MS) and cold coma (CC) were examined decreasing the aquarium temperature in average 1.15 ± 0.14°C min-1 . For the fish thermal tolerance studies, we applied the model of Kilgour et al. (1985) to asses the upper incipient lethal temperature (UILT). The experimental temperatures where 50% of the mortality occurred were established, based on the temperatures where the CTMax behavior responses (MS and LE) were observed and, 1°C was added or subtracted to the CTMax (Table 3). Ten organisms of each TF cycle were placed in each aquarium were temperature remain constant for 48 h experimentation. The hour that each fish entered the aquariums and their moment of death was registered (Table 3).


To study the low incipient lethal temperature (LILT), a horizontal acrylic tank was used (describe in Bückle et al., 2003) conditioned with 5-20°C thermal gradient established based on the temperatures in which the CTMin responses (SA+MS and CC) were observed. Five to six experimental temperatures were determined for each TFs cycle (Table 3). Each experimental temperature was represented by eight fish confined inside the gradient in perforated plástic baskets and placed in the chamber matching the experimental temperature. The gradient temperature was registered with a thermograph (Stanford Research Systems, Mod. SR 630) every two minutes during the 45 min. The LILT was established when coma response took place, and immediately after, fish were returned to perforated baskets in the corresponding temperature fluctuation tank to evalúate their recovery during the following 48 h.

The preferred temperature (PT) or final preferendum and the low and high avoided temperatures (LAT and HAT) were studied with the acute method in the same horizontal acrylic thermal gradient. In previous experiments we observed thermal preference among sexes (Hernández-Rodríguez et al., 2002). Therefore 10 males or females separately were chosen at the moment of maximum temperature of the 29 or 35°C TFs exposure and placed in the chamber of the thermal gradient matching the same temperature. The trials began 40 min later to reduce handling stress. Temperature and organisms location in the gradient cameras was registered every 10 min during two hours. The LAT and HAT were estimated with the temperature less frequented by fish throughout the experiment.

Data analyses were performed using SIGMASTAT Versión 3.1 software (SPSS, Chicago, IL, USA). The analysis of the results was made with the variance (ANOVA) test at a significance level a = 0.05. The data didn't have a normal distribution, non parametric tests of variance analysis were applied by ranges (Kruskal-Wallis) and múltiple comparisons (method of Dunn and Student-Newman-Keuls) (Zar, 1984).

RESULTS

The responses LE and MS were significantly different (P < 0.001) between the thermal fluctuations 20-29 and 26-3 5°C. The fish acclimated to the 26-3 5°C thermal fluctuation increase the responses MS and LE in 2°C. There exist no significant differences when LE and MS responses are contrasted between cycles of the same TF (Fig. 2a).


 

Figure 2. Relationship among the high (a) and low (b) temperatures that caused the stress responses in Poecilia sphenops acclimated to 20-29 and 26-3 5°C thermal fluctuations of the symmetrical cycle and asymmetric cycle. Muscular spasms (MS), loss of equilibrium (LE), sporadic activity with muscular spasms (SA+MS) and, cold coma (CC). Each valué corresponds to the average of four repetitions (± standard deviation).

Figura 2. Relación entre las temperatures altas (a) y bajas (b) que causaron las respuestas de estrés en Poecilia sphenops aclimatada a las fluctuaciones térmicas 20-29°C y 26-3 5°C del ciclo simétrico y asimétrico. Espasmos musculares (MS), pérdida de equilibrio (LE), actividad esporádica con espasmos musculares (SA+MS) y estado de coma al frío (CC). Cada valor corresponde al promedio de cuatro repeticiones (± la desviación estándar).

Fish responses exposed to CTMin were significantly different (P < 0.05, Dunn's Method test). The SA+MS and CC responses of fish acclimated to the cycles of 20-29°C and 26-3 5 °C TFs were statistically different. Fish acclimated to the SC and AC of the 26-3 5 °C TF were less resistant to cold in 1.3-1.6°C compared to the organisms exposed to the 20-29°C TF (Fig. 2b). Similar temperatures were observed for the SA+MS and CC responses, and the differences were less than 0.6°C (Fig. 2b). The upper incipient lethal temperature (UILT) values of the fish acclimated to both cycles and TFs had a thermal interval of 38.8 to 39.5°C (Table 4a).


The low incipient lethal temperature (LILT) of fish exposed to the 20-29°C and 26-35°C TFs differ in only 1°C (Table 4a), the thermal fluctuation do not affect the low temperature responses. However, organisms in the SC and AC of the 20-29°C exposed to the bath temperature of 5.5 and 6.2°C died. Most fish in the SC and AC of the 26-35°C that were abruptly exposed to cold bath temperatures, a sanguine spill in the mouth and others, a corporal bend took place and fish mortality was 62.5% and 66% when exposed to 9.8°C bath temperature.

Females preferred temperature around 30°C in both TFs was not significantly different (P < 0.001). Males preferred temperature (mean 28°C) increase by the effect of the AC of the TFs compare to the SC (mean 24°C) (Table 4a).

The thermal fluctuations modified the fish avoided temperature interval delimited by LAT and HAT. In females and males acclimated to 20-29°C was more than 14°C, and in the 26-3 5°C cycle the interval decrease in 4°C and 6.7°C respectively (Table 4a).

DISCUSSION

The thermal fluctuations, each one, with an asymmetric and symmetrical cycle provided important knowledge over the resistance and tolerance of P. sphenops. The critical thermal maximum used as a stress and adaptation indicator, in this species is characterized by the muscular spasm according to the analysis carried out by Lutterschmidt and Hutchison (1997). The vicinity among the responses of muscular spasm and loss of equilibrium represents the thermal transition area that allows the organisms to escape from the stress caused by high temperatures and it constitutes the threshold of thermal resistance.

P. sphenops acclimated to the 20-29°C and 26-35°C TFs increased their resistance in 2°C compared with the results of the constant temperatures experiments 20 to 35°C, where we observed that for each 3°C increment, CTMax increase 1°C, and it was constant in 32 and 35°C (Hernández-Rodríguez & Bückle-Ramirez, 1998). In ecological terms, it represents the thermal response capacity of the species to temperarme variations and only when acclimated to constant temperatures we can establish his resistance limit and the relative change in °C. P. sphenops that experience great thermal fluctuations in the environment of the dam is capable to increase their resistance in 3.0°C (AC of the TFs) (Table 4).

The responses of increased activity+muscular spasm and cold coma of fish acclimated to the thermal fluctuation had very similar values and the transition among the temperatures of these responses is 2.0°C, which allows them to escape from the stress caused by the decrease of the environmental temperature, before the cold coma takes place. Fish acclimated to the thermal fluctuations did not resist temperatures under 10°C. In this study fish exposed to cold did not die after they lost the equilibrium and remained in lethargy, and the time in stupor was fundamental for their recovery, when returned to the fluctuating acclimation temperatures. P. sphenops acclimated to the thermal fluctuations experienced a primary coma according to the description of Pitkow (1960) as a result of the continuing temperature decrease, where the time of exposure was decisive for survival and the immediate recovery.

There are few works related with the organism's critical thermal minimum characterized by the loss of equilibrium (Becker & Genoway, 1979) or when the animals enter in coma (Pitkow, 1960). The values of the critical thermal minimum that Barrionuevo & Fernandes (1995) obtained in Prochilodus scrofa acclimated at 15-35°C were 5-14.5°C. In P. sphenops acclimated to the 20-29°C and 26-35°C TFs; the cold coma occurred at a mean temperature of 10.3 and 11.8°C respectively. However, when P. sphenops was acclimated to constant ambient of 20 to 26°C the TCMin was 7.7 to 9.9°C (Hernández-Rodríguez & Bückle-Ramirez, 1998).

In this species, prolonged exposures to low constant acclimation temperature (20°C), cold resistance increase 2.5°C (Tables 4a and 4b).

The increasing resistance in P. sphenops after exposed to the critical thermal maximum and minimum experiments subsequent to the thermal fluctuations reveáis an adjustment of the fish homeostatic mechanisms reflected in their capacity of thermal response. The thermoresistence of fish exposed to increasing and decreasing temperatures can be consider as an approach to select more resistant species to the environmental thermal variations.

The daily and seasonal temperature fluctuations influence the critical thermal maximum temperature evidencing the differences and adjustments in the tolerance to high temperatures and in the distribution of several species such as Etheostoma flabellare, E. blennoides, E. caeruleum and Morone saxatilis (Hlohowskyj & Wissing, 1985; Cox, 1978). The acclimation capacity of the organisms confronting different temperature regimens differs among species, which indicates that the thermal ability can be correlated with the environmental thermal stability (Barrionuevo & Fernandes, 1995).

Currie et al. (2004) found that CTMax and CTMin of Ictalurus punctatus, Micropterus salmoides and Oncorhynchus mykiss exposed for 24 h to a sine-wave trajectory "10°C thermo period" for 32 consecutive days, did not reveal "increase or decrease differences between the same species acclimated to a constant temperature equal to the mean of the thermoperiod". Their results differ from other research therefore they consider that "the explanation may be the characteristics of thermo periods including shape, peak, minimum temperature and amplitude". We agree that thermo cycles characteristics are very important for analyzing, comparing and explaining results but also associate with the fact that this type of response is species specific.

The upper incipient lethal temperature of P. sphenops exposed to the 20-29 and 26-35°C TFs had an interval of 38.8-39.5°C, these values are similar when this species is acclimated at constant temperatures, 26 to 35°C (Table 4b) (Hernández-Rodríguez, 1998). Similar results have been obtained in Salvelinus fontinalis, Cyprinodon nevadensis, Salmo gairdneri and Lepomis gibbosus, where the resistance to extreme temperatures was associated with the thermal history or with the fish acclimation conditions (Brown & Feldmeth, 1971; Fahmy, 1972; Hokanson, 1977; Becker & Genoway, 1979).

The lower incipient lethal temperature of P. sphenops acclimated to both thermal fhictuations was 10.8-11.8°C and they do not resist less than 10°C, however, they increased its resistance in 0.67 and 0.95°C respectively. The thermal differences of this response were of 3.3 and 1.0°C compare with fish acclimated firom 20 to 26°C constant temperatures (7.5-9.8°C) respectively. In this species, the acclimation to constant and fluctuating temperatures is a determining factor to identify the thermal points that characterize this type of response. This response has been studied in few species, and the values obtained are dependent of the thermal history and the acclimation temperature. The LILT values varied depending on the methodology used by the authors that can imply, an abrupt thermal change or gradual decrease of 1°C day-1 until the organisms loses the equilibrium or they fall in coma.

The results of experiments with the low and high lethal temperatures vary due to the grade of eurythermality and stenothermality among species, because the organisms resistance depends on the thermal history (acclimation or acclimatization), age and time-space distribution. Fry (1967) mentioned that lethal temperatures are important for the physiologic analysis of poikilotherm organisms because it provides a detailed pattern of responses that can be used as an immediate ecological index, since the animals can find lethal temperatures in the habitat as well as thermal fluctuations overcoming their tolerance limits.

Fish behaviour studies, included in the tolerance area of the response polygon; consider the preferred temperature as one of the most important responses because the organisms choose the temperatures that could be the appropriate for their physiologic processes efficiency. In P. sphenops acclimated a constant and fluctuations temperature, the thermal preference was influenced by the sex (Tables 4a and 4b) (Hernández-Rodríguez et al., 2002). Females had a final preferendum of 31.6°C during the day and 28.9°C in the night. Males preference was 26.9°C during the day and 25.5°C in the night (Hernández-Rodríguez, 1998).

In general, females of P. sphenops prefered higher temperatures than males which could be associated with the grade of sexual maturity studied in other species. For instance, in Gambusia sp. and Dionda nubila has been pointed out that the resistance to heat may be concatenated with sex and it is suggested that, fish sexual hormones could be related with the thermoregulation behaviour (Hagen, 1964; Baker et al., 1970). In this conduct Johansen and Cross (1980), studied the effect of steroid hormones on the sexual maturation of Poecilia reticulata that influenced the males preferred temperature by choosing lower temperatures (24.5°C) than females (28.2°C) and juveniles (28.1°C). The authors also comment that the male's androgenic hormone induces the preferred temperature decrease, which was verified treating juveniles and females with several testosterone concentrations. A similar situation in P. sphenops is probable; however, it is necessary more studies in this theme.

For many species, the preferred temperature is mainly a function of the thermal acclimation history of the individuáis. In this context, the preferred temperature varíes markedly among the species in direct or inversely proportional correspondence to the acclimation temperature. This correlation is an adaptative response, where the exposure to a new temperature causes a compensatory physiologic adjustment by increasing the metabolic efficiency to the new condition generating an appropriate adjustment in the preferred temperature (Kelsch & Neill, 1990).

The thermoregulatory behaviour of the organisms is extremely variable, because of the numerous factors like age, sex, time of the year and food availability, among others that influence the organism's response in such a way that, each species is characterized by a preferential interval or a final preferendum where the biological processes are at their maximum efficiency. When the eurythermic species behaviour is associated with the geographical distribution the panorama of influences is expanded with ecological, social and genetic factors. On the other hand in the stenothermic species, the temperature effect at physiologic level can be the dominant factor that governs its distribution (Cherry et al., 1975, 1977).

The preferential orientation that the organisms use to select a thermal interval in the environment, serves them to find and adapt to the temporary (provisional) space of their niches. In this theme, the response of thermal avoidance of P. sphenops that were acclimated to the 20-29°C TF had an avoidance range over 14°C; and in the symmetric and asymmetric cycles of 26-35°C TF the avoided temperaturas had an interval of 9.3 and 11.4°C respectively. The fish acclimated to the TFs of the SC and AC enlarged the avoidance interval. Most of the investigations related with the avoided temperatures have been made in organisms acclimated to constant temperatures. Eurythermic species like the ciprinidae, centrarquidae and ictaluridae acclimated firom 6-3 0°C are characterized for having relatively wide intervals of low and high avoided temperatures (Cherry et al., 1975, 1976, 1977). Jander (1975) based on the lifestyle of the species, considers that the organisms that minimize the effective distance of a primary resource (food, refuge, reproduction and other) they maximize their effective distance of a stress source (temperature, predators and other) they can respond better to the environmental variation compare with other animals that do not use this mechanism.

The capacity of adaptation to thermal changes is a highly complex problem, because the physiologic adjustments of the organisms happen at all levéis of its biological organization with compensatory and other non-compensatory responses, the latter usually associated to seasonal rhythms as, food availability, reproduction, migration and molt, among other (Cossins & Bowler, 1987).

The knowledge of the relationship between the constant or fluctuating acclimation temperature and the thermoregulatory behaviour of the species allows understanding part of the biology, geographical distribution and adaptation capacity of P. sphenops to the thermal changes that can occur in the habitat. This ability could explain the wide geographical distribution of P. sphenops and the high reproduction rate. The behaviour pattern observed in this investigation including the results of the constant temperatures experiments, characterizes it as a species with a great plasticity response towards the thermal fluctuations that occurred in the dam with a daily monthly average maximum temperature from 26 to 36°C and minimum from -0.2 to 5°C. P. sphenops is perfectly adapted to the marked rain and drought seasons of their habitat, via increasing or decreasing their preference intervals, tolerance and resistance conferring them a better adjustment capacity towards the temperature variation.

 

ACKNOWLEDGEMENTS

This work was supported by the Federal Government of Mexico, through financing of the Center of Scientific Research and High Education of Ensenada (CICESE) and the project 225080-5-1882PB of Co.Na.C. y T. We extend our gratefulness to Dr. Victor A. Zavala Hamz for the English review and Francisco Valenzuela Buriel for the elaboration of the figure.

 

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Corresponding author: Mónica Hernández Rodríguez (mhernand@cicese.mx)

Received: 24 March 2009; Accepted: 6 August 2010